A capacitor element 1 to be built in a solid electrolytic capacitor such as a conventional tantalum electrolytic capacitor is fabricated as follows. As shown in FIG. 3, by using, for example, Ta powders, a molded body of a rectangular parallelepiped or a cube shape in which an anode lead 11 such as a Ta wire is embedded in one of the wall faces is obtained. The molded body is sintered in a vacuum atmosphere. Around the powders and the sintered body 10, a dielectric layer (oxide film) 14 made of Ta.sub.2 O.sub.5 or the like and a manganese dioxide layer 15 are formed. Further, a graphite layer 16, and a silver resin layer 18 are sequentially formed, thereby obtaining an enveloping layer 19. The surface of the enveloping layer 19 is used as a cathode 12. The dielectric layer 14 is formed by anodizing (conversion treatment(coating)) the sintered body 10. The manganese dioxide layer 15 is formed by soaking the sintered body 10 in a manganese nitrate solution and thermal decomposing the manganese nitrate solution. The graphite layer 16 is formed by applying aqueous graphite (what is called India ink) and sintering it at about 130.degree. C. The silver resin layer 18 is formed by applying what is called a silver paste and drying it at about 200.degree. C. Reference numeral 13 denotes a Teflon ring.
As described above, the cathode 12 of the capacitor element 1 is formed by providing the enveloping layer 19 consisting of the above layers around the sintered body 10. The coefficients of linear expansion of the layers are different from each other as shown in Table 1.
TABLE 1 Coefficients of linear expansion of layers Coefficient of material linear expansion manganese dioxide layer 0.6 .times. 10.sup.-5 graphite layer 0.3 .times. 10.sup.-5 silver resin layer 4.0 .times. 10.sup.-5
As described above, in the cathode of the capacitor element, the various layers are provided on the surface of the sintered body on which the dielectric layer is formed and their coefficients of linear expansion are different from each other. Consequently, by a stress of a temperature shock of solder reflow at the time of mounting the solid electrolytic capacitor on a circuit board or the like, each of the layers is peeled or floated. As a result, the impedance (equivalent series resistance at a high frequency) increases and a problem such as deterioration in electric characteristics arises.